Sodium cyanide (NaCN) has a variety of uses. For example, it is used in electroplating, treating metal surfaces, extracting and recovering precious metals from ores, and a number of other chemical applications. The use of NaCN in the leaching of ores that contain precious metals, such as gold and silver, is especially well known in the art.
The production of NaCN for the above use is accomplished by the so-called "wet" process, wherein hydrogen cyanide (HCN) is neutralized with aqueous sodium hydroxide (NaOH) solution, followed by evaporative crystallization to produce a slurry of NaCN crystals. The NaCN crystals are separated from the slurry, dried, and usually formed by dry compression methods into briquettes. The resulting briquettes are about 99 weight percent anhydrous NaCN.
The briquettes are shipped to consumers generally in containers designed to exclude exposure to atmospheric air since the anhydrous NaCN is very hygroscopic and can absorb substantial quantities of water when exposed to atmospheric air. If exposed to atmospheric air, serious difficulties in shipping and storage can result due to caking. Also, there is the added cost of excluding atmospheric air.
The majority of consumers generally convert the NaCN briquettes into an aqueous solution. They dissolve the NaCN, sometimes after breaking the briquettes into smaller particles, to produce a solution containing about 20 to 25 weight percent NaCN. To avoid hydrolysis with the resulting evolution of hazardous hydrogen cyanide vapors, an alkali such as NaOH is added to the dilution water. Sufficient alkali should be added to raise the resulting solution pH to about 12 or higher.
To avoid difficulties and cost associated with storage of the anhydrous product and industrial hygiene hazards due to generation of respirable dust when handling anhydrous NaCN briquettes or breaking the briquettes into smaller particles, some of the larger consumers have changed to direct solution storage. This has been accomplished by dissolution of the briquettes in the shipping container, usually tank trucks or railway cars, or in a storage tank, and unloading the resulting solution into storage tanks.
The manufacture of briquettes has the disadvantages of added investment and operating cost associated with concentrating, separating, drying and compacting anhydrous NaCN only to dissolve and dilute it prior to use. It would thus appear that direct shipment of NaCN solution, particularly where the NaCN production facilities are located in reasonable proximity of the consumer, would be highly desirable. However, the shipment of NaCN solution presents high shipping costs and a high environmental risk of spills in the event of an accident during transportation.
Anhydrous crystals of NaCN can be prepared by a number of processes known in the art. For example, McMinn, U.S. Pat. No. 2,708,151 and Oliver, U.S. Pat. No. 2,726,139, disclose processes for reacting substantially pure HCN with substantially pure NaOH to form solutions containing NaCN. Mann, et al., U.S. Pat. No. 3,619,132; Cain, U.S. Pat. No. 2,616,782; Mittasch, et al., U.S. Pat. No. 1,531,123; and Rogers, et al., U.S. Pat. No. 4,847,062 disclose processes for preparing NaCN employing impure starting materials. Isolating dry crystals of NaCN from reaction solution presents the hazards of exposing to harmful dust of anhydrous NaCN and additional manufacturing costs.
Accordingly, there is a need for a NaCN composition that can be inexpensively produced and packaged in sufficiently high NaCN weight percentage concentrations, with low risk to the environment when shipped over distances, and efficiently dissolved into solution upon delivery to the consumer. There is also a need to develop a process for producing a NaCN that eliminates the need to separate, dry and compact the NaCN prior to shipment by providing a composition with a paste-like consistency which is highly desirable from the standpoint of safe transport of product to consumers.
An advantage of the present invention is that the NaCN concentration in the paste composition is sufficiently high that the cost of shipping water does not become an overriding concern. Another advantage of the present invention is that the paste composition of this invention eliminates for consumers the hazards of exposure to harmful dusts associated with handling the anhydrous NaCN briquettes, and the need to add caustic to avoid the generation of hazardous HCN vapors due to hydrolysis of the NaCN. Also an advantage of the paste compositions of this invention is that the paste incorporates sufficient base and, therefore, consumers may not need to provide additional base to avoid hydrolysis, when used in an application. Other advantages will become more apparent as the invention is more filly disclosed hereinbelow.